The direct reduction of iron oxide materials with solid carbonaceous reducing agents can reduce the materials containing iron oxide in a first fluidized bed vessel containing an expanded fluidized bed and supplied with solid carbonaceous reducing agents and fluidizing gas and operated under weekly reducing conditions with such a short residence time of the gas in the fluidized bed vessel that the reduction potential results in a reduction to FeO or to less than the FeO state. The gas-solids suspension discharged from the first fluidized bed vessel containing an expanded fluidized bed can be fed to a second fluidized bed vessel containing an expanded fluidized bed to which a highly reducing gas is fed as a fluidizing gas.
A strongly reducing exhaust gas and a major part of the resulting carbonized carbonaceous material can be discharged from the upper portion of the second fluidized bed. The carbonized carbonaceous material can be separated from said exhaust gas and recycled to the first fluidized bed vessel. The remaining exhaust gas can be purified and CO.sub.2 can be removed therefrom and subsequently a part of the gas can be recycled as the highly reducing fluidizing gas to the second fluidized bed vessel.
Reduced material which has been metallized to a degree from 50 to 80% and the remainder of the carbonized carbonaceous material can be discharged from the lower portion of the second fluidized bed vessel. The carbonized carbonaceous material which is recycled to the first fluidized bed vessel is generally recycled at a rate which is a multiple of the rate at which the iron oxide-containing materials are charged to the first fluidized bed vessel. The heat content of the suspension passed from the first fluidized bed vessel to the second is usually sufficient to supply the second fluidized bed vessel with the heat required to be consumed therein.
In the winning of ores and the processing of relatively low grade iron ores, fine-grained ores become available in progressively increasing quantities. Such fine-grained ores can be directly reduced at temperatures below their melting temperature in special fluidized bed processes, which are carried out under highly favorable mass and heat transfer conditions.
In the direct reduction with solid carbonaceous reducing agents, the following different processes must consecutively be performed: Heating up of ore and coal, low-temperature carbonization of the coal, gasification of the coal, reduction of the ore and an optional cooling of the end product.
European Patent Publication 255,180 discloses the process described above for the direct reduction of fine-grained materials containing iron oxide in two fluidized bed vessels, which are connected in series. In preferred embodiments the inlet temperature of the suspension passing from the first fluidized bed vessel into the second is 30.degree. to 80.degree. C. above the temperature of the gas-solids suspension withdrawn from the upper portion of the second fluidized bed vessel and the carbonized carbonaceous material is circulated at a rate which is 10 to 50 times the rate at which the iron oxide-containing material is charged.
The carbonized carbonaceous material is separated from the material which has been discharged from the lower portion of the second fluidized bed vessel. At least part of the separated material is recycled to the first fluidized bed vessel. The iron oxide-containing material before being charged to the first fluidized bed vessel is preheated by the highly reducing gas from the second fluidized bed vessel.
The reduced material from the second fluidized bed vessel is completely reduced and melted in melting reactors and the exhaust gas from the melting reactors is fed as a highly reducing gas to the second fluidized bed vessel. Part of the separated carbonized carbonaceous material is charged to the melting reactor.
That process affords the advantage that hot spots and encrustation particularly at the nozzles for supplying oxygen-containing gases, will be avoided in the direct reduction with solid carbonaceous reducing agents in fluidized beds and that the reduction in the fluidized beds can be effected without disturbance.
There is virtually no metallic iron in the first fluidized bed vessel, which is supplied with oxygen-containing gases to produce the required heat. As a result, a reoxidation of iron metal and hot spots at the nozzles are avoided.
Such reoxidation and hot spots will also be avoided in the second fluidized bed vessel, which is not supplied with oxygen-containing gases. A recycling of iron metal from the second fluidized bed vessel to the first fluidized bed vessel will also be avoided.